使用KVO跟踪NSProgress

Question

我正在使用KVO跟踪正在接收的文件的进度：

-(void)session:(MCSession *)session didStartReceivingResourceWithName:(NSString *)resourceName fromPeer:(MCPeerID *)peerID withProgress:(NSProgress *)progress

{
NSLog(@"RECEIVING... %@ from peer: %@", progress, peerID);
dispatch_sync(dispatch_get_main_queue(), ^{
    [progress addObserver:self
                forKeyPath:@"fractionCompleted"
                   options:NSKeyValueObservingOptionNew
                   context:NULL];
});}

和

- (void)observeValueForKeyPath:(NSString *)keyPath
                  ofObject:(id)object
                    change:(NSDictionary *)change
                   context:(void *)context
{
if (object == progress) {
    // Handle new fractionCompleted value
[progressBar setProgress:progress.fractionCompleted animated:YES];
    NSLog(@"Fraction Complete: %@", [NSNumber numberWithDouble:progress.fractionCompleted]);
    return;
}

[super observeValueForKeyPath:keyPath
                     ofObject:object
                       change:change
                      context:context];
}

我想用这个来更新一个UIprogressView.但是代码在这一行上崩溃了，我不明白为什么：

[super observeValueForKeyPath:keyPath
                     ofObject:object
                       change:change
                      context:context];

编辑：

这是错误

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason:     '<ViewController1: 0x15b57000>: An -observeValueForKeyPath:ofObject:change:context: message was     received but not handled.

编辑：

如果删除超级观察者observeValueForKeyPath:keyPath.，应用程序不会崩溃，但是NSLog(@“分数完成：%@总是报告值"0”)。

Answer 1

您的超类有observeValueForKeyPath:ofObject:change:context:实现吗？您可以对此进行一次if检查：

if ([super respondsToSelector:@selector(observeValueForKeyPath:ofObject:change:context:){
    NSLog(@"Yeah, I'm here matey!");
} else {
    NSLog(@"Uh oh said the selector ghost");
}